1. Field of the Invention
This invention relates to a process for recovery of chemicals from a pulping waste liquor, and more particularly to a process for recovery of chemicals from a smelt which is obtained by combustion of concentrated pulp cooking waste liquor containing a sulfite or a bisulfite.
2. Description of the Prior Art
Various processes for producing pulp from cellulosic material, for example, wood chips have been practiced. Among them, more interest has recently been drawn to a process in which the cooking chemicals are sodium sulfite or sodium bisulfite in combination with sodium carbonate, because cellulosic pulp is produced in high yield to make the process economical. However, in this process no effective commercial recovery system of chemicals from the waste liquor has been established.
In Japanese Pat. No. 14401/74, we proposed a system for recovery of chemicals comprising concentrating a cooking waste liquor from sodium sulfite process, burning the concentrate to obtain a smelt mainly comprising sodium sulfide and sodium carbonate, oxidizing the smelt with air to convert sodium sulfide into sodium sulfite, dissolving the oxidized material in water and treating the resulting aqueous solution with a sulfur dioxide-containing gas to convert sodium carbonate into sodium sulfide, thereby regenerating an aqueous solution which can be used as a cooking liquor in the pulping process.
Although this prior process deals with a fundamental, technical concept for recovering chemicals from sulfite pulping waste liquor, there are still many problems to be solved. For example, the smelt contains, in addition to sodium sulfide and sodium carbonate, a small amount of sodium sulfate and slight amounts of sodium thiosulfate, sodium sulfite and sodium chloride, some of which are hard to convert into the desired chemicals, and the proportion of various chemicals present in the smelt may vary over a wide range depending upon the cooking conditions employed in the particular pulp mill; accordingly, it is difficult to standardize the conditions under which the recovery of chemicals is practiced and to design an apparatus suitable for carrying out the process.
Accordingly, an object of this invention is to provide a process for recovery of chemicals from sulfite pulping waste liquor without causing environmental pollution and any appreciable loss of chemicals.
When a typical waste liquor from semichemical sulfite process is concentrated and burned, the resulting smelt has the following composition ranges (by weight):
______________________________________ Na.sub.2 S 30 to 40% Na.sub.2 CO.sub.3 45 to 60% Na.sub.2 SO.sub.4 5 to 10% Na.sub.2 S.sub.2 O.sub.3 0-5% others 2-4% ______________________________________
It has already been known that, when smelt is treated with a molecular oxygen-containing gas in the presence of water, the sodium sulfide and sodium sulfite are oxidized according to the following reaction formulae: EQU 2Na.sub.2 S+30.sub.2 .fwdarw.2Na.sub.2 SO.sub.3 ( 1) EQU Na.sub.2 S+20.sub.2 .fwdarw.Na.sub.2 SO.sub.4 ( 2) EQU Na.sub.2 SO.sub.3 +1/2O.sub.2 .fwdarw.Na.sub.2 SO.sub.4 ( 3) EQU 2Na.sub.2 S+H.sub.2 O+20.sub.2 .fwdarw.Na.sub.2 S.sub.2 O.sub.3 +2NaOH (4)
Reaction (1) occurs at a relatively low temperature and reactions (2) and (3) at relatively high temperature.
In addition to the reactions mentioned above, the following side reactions concurrently occur: EQU Na.sub.2 S.sub.2 O.sub.3 +2NaOH.fwdarw.3/4Na.sub.2 SO.sub.3 +2/3Na.sub.2 S+H.sub.2 O (5) EQU Na.sub.2 S.sub.2 O.sub.3 +2NaOH+O.sub.2 .fwdarw.2Na.sub.2 SO.sub.3 +H.sub.2 O (6)
On the other hand, the sodium carbonate which is one component of the smelt is unchanged during the oxidation treatment.
Thus, in general, such oxidation treatment involves various reactions and the primary purpose is to convert the sodium sulfide into sodium sulfite and to prevent the formation of sodium sulfate and sodium thiosulfate which are inactive in the pulping process. However, in practice, it is difficult to control the oxidation treatment to such an extent that only the desired reaction (1) will occur.
Further, an aqueous solution of the oxidized product is treated with a sulfur dioxide-containing gas to convert the sodium carbonate into sodium sulfite. The source of said sulfur dioxide is, in general, an exhaust gas from the recovery boiler and substantially all of the sulfur dioxide released during the combustion of concentrated waste liquor is recovered by being absorbed in the aqueous solution to regenerate a cooking liquor whereby the overall process can be operated as a closed system.
Accordingly, in order to successfully carry out the process, it is essential that, in the mixture to be oxidized, the molar ratio of S/Na.sub.2 O be maintained substantially equal to that of the smelt and the water content be kept at an appropriate level.
In general, the temperature at which the oxidation is effected is controlled by adjusting the amount of water contained in the reaction mixture, consequently, by adjusting the amount of heat removed from the mixture by evaporation of water, so that the reaction temperature is maintained within a range within which sodium sulfide is effectively converted into sodium sulfite.